1. Field of the Invention
The present invention is generally directed toward improved methods for making gas diffusion electrodes for electrochemical devices, and more particularly, to methods of achieving a smoother surface of the gas diffusion electrodes at an ion-exchange membrane interface.
2. Description of the Related Art
Electrochemical cells comprising ion exchange membranes, such as proton exchange membranes (PEMs), for example, polymer electrolyte membranes may be operated as fuel cells, wherein a fuel and an oxidant are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers, wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes. FIGS. 1-4 collectively illustrate a typical design of a conventional membrane electrode assembly 5, an electrochemical cell 10 comprising a PEM layer 2, and a stack 50 of such cells.
Each cell 10 comprises a membrane electrode assembly (“MEA”) 5 such as that illustrated in an exploded view in FIG. 1. The MEA 5 comprises an ion exchange membrane 2 interposed between first and second electrode layers 1, 3, which are typically porous and electrically conductive. As illustrated in FIG. 5, the electrode layers 1, 3 typically comprise a gas diffusion layer 20, 22 and an electrocatalyst 21, 23 positioned at an interface with the ion-exchange membrane 2 for promoting the desired electrochemical reaction.
In an individual cell 10, illustrated in an exploded view in FIG. 2, an MEA 5 is interposed between first and second cell separator plates 11, 12, which are typically fluid impermeable and electrically conductive. The cell separator plates 11, 12 are manufactured from non-metals, such as graphite; from metals, such as certain grades of steel or surface treated metals; or from electrically conductive plastic composite materials.
Electrochemical cells 10 with ion exchange membranes 2 are advantageously stacked to form a stack 50 (see FIG. 4) comprising a plurality of cells disposed between first and second end plates 17, 18. A compression mechanism is typically employed to hold the cells 10 tightly together, to maintain good electrical contact between components, and to compress the seals. In the embodiment illustrated in FIG. 3, each cell 10 comprises a pair of cell separator plates 11, 12 in a configuration with two cell separator plates per MEA 5. Cooling spaces or layers may be provided between some or all of the adjacent pairs of cell separator plates 11, 12 in the stack 50. An alternate configuration (not shown) has a single separator plate, or “bipolar plate,” interposed between a pair of MEAs 5 contacting the cathode of one cell and the anode of the adjacent cell, thus resulting in only one separator plate per MEA 5 in the stack 50 (except for the end cell). Such a stack 50 may comprise a cooling layer interposed between every few cells 10 of the stack, rather than between each adjacent pair of cells.
The illustrated cell elements have openings 30 formed therein which, in the stacked assembly, align to form gas manifolds for supply and exhaust of reactants and products, respectively, and, if cooling spaces are provided, for a cooling medium.
The gas diffusion layers 20, 22 of the respective electrode layers 1, 3, illustrated in FIG. 5, comprise a substrate with a porous structure, which renders it permeable to fluid reactants and products in the fuel cell.
Fluid reactants may be supplied to the electrode layers 1, 3 in either gaseous or liquid form. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) and electrons from the fuel. The gaseous reactants move across and through the gas diffusion layer 20 to react at the electrocatalyst 21. The ion-exchange membrane 2 facilitates the migration of protons from the anode 1 to the cathode 3 while electrons travel from the anode 1 to the cathode 3 by an external load. At the electrocatalyst layer 23 of the cathode 3, oxygen reacts with the protons that have crossed the membrane 2 and the electrons to form water as the reaction product.
The electrocatalysts 21, 23 are typically disposed in a layer at each interface between the membrane 2 and the gas diffusion layers 20, 22, to induce the desired electrochemical reaction in the MEA 5. The electrocatalysts 21, 23 may be a metal black, an alloy or a supported catalyst, for example, platinum on carbon. The catalyst layers 21, 23 typically contain an ionomer, which may be similar to that used for the ion-exchange membrane (for example, up to 30% by weight NAFION® brand perfluorosulfonic-based ionomer). The catalyst layers 21, 23 may also contain various binders, such as polytetrafluoroethylene (PTFE). The electrocatalysts 21, 23 may be disposed as a layer on the gas diffusion layers 20, 22 to form the electrode layers 1, 3 or disposed as a layer on the ion-exchange membrane 2.
Materials commonly used as gas diffusion layers 20, 22 or as starting materials to form gas diffusion layers 20, 22 include carbon fiber paper, woven and nonwoven carbon fabrics, metal mesh or gauze, and other woven and nonwoven materials. Such materials are commercially available in flat sheets and, when the material is sufficiently flexible, in rolls. Gas diffusion layers 20, 22 tend to be highly electrically conductive and macroporous and may also contain a particulate electrically conductive material and a binder. The substrate may be pre-treated with a water-repellent material, for example, a fluororesin such as polytetrafluoroethylene, or with a mixture of a water repellent material and an electrically conductive material, such as a fluororesin and carbon black, to enhance water repellency. The gas diffusion layers 20, 22 may also comprise a sub-layer (typically comprising an electrically conductive material, such as carbon and graphite, and/or a water-repellent material, such as a fluororesin, or a mixture thereof coated on one side thereon in order to reduce porosity, provide a surface for electrocatalysts 21, 23, reduce surface roughness or achieve some other objective. The sub-layer can be applied by any of the numerous coating, impregnating, filling or other techniques known in the art. For example, the sub-layer may be in the form of an ink or paste that is applied to the substrate and may at least partially penetrate into the substrate.
In preparing the electrode layers 1, 3, it has been found desirable to reduce the surface roughness. Reduced surface roughness allows the electrode layers 1, 3 to sustain an improved bond with thin ion-exchange membranes 2. Peaks in the surface of one or both electrode layers 1, 3 may lead to perforations or leaks in ion-exchange membranes 2 when assembled into an MEA 5. The electrode layers 1, 3 may cause perforations or leaks by penetrating the ion-exchange membrane 2 or by reducing the thickness of the ion-exchange membrane 2. Pores or depressions in the surface of the electrode layers 1, 3 may also cause leaks, for example, as compressive stresses cause the membrane 2 to flow into pores and other surface depressions when the MEA 5 is heated, such as during bonding and fuel cell operation.
Accordingly, a smoother surface of the electrode layers 1, 3, reduces the quantity and severity of cracks in the electrode layers 1, 3, and in the ion-exchange membrane 2, preventing adverse fluid leaks that may result in fuel waste, corrosion, or mass transport losses. Examples of such cracks are shown in the optical images 24 of FIGS. 6-9. FIG. 10, which is a scanning electron micrograph 26 of a cross-section view of the electrode of FIG. 7, further reveals the inconsistencies and incontiguities in existing electrode layers 1, 3.
Various methods of assembling MEAs 5 have been proposed to reduce the surface roughness of the electrode layers 1, 3 at the ion-exchange membrane 2 interface. These methods typically include controlling the thickness of the electrocatalyst as described in U.S. Publication No. 2004/0209138, using roller compaction as elicited in U.S. Publication No. 2002/0192383, using heated rollers as explained in Japanese Publication No. 2001-038157, and compacting of multi-layered catalyst layers as provided in International Publication No. WO 02/089237. A further proposed method of producing MEAs 5 includes drying the catalyst ink during compaction. Yet other proposed methods include electrostatically charging the electrocatalysts 21, 23 prior to application onto the ion-exchange membrane 2 or to the gas diffusion layers 20, 22 of the electrode layers 1, 3.
Although the above-described methods have introduced limited reduction of surface roughness of the electrode layers 1, 3 at the ion-exchange membrane 2 interface, obstacles persist. For example, surface roughness continues to present difficulties in bonding the membrane 2 to the electrode layers 1, 3. Additionally, cracks, similar to those in FIGS. 6-9, continue to emerge, resulting in inefficient fluid diffusion, fuel waste, mass transport losses and corrosion of the MEA 5 components. Furthermore, processes such as hot compaction are expensive and time-consuming. Other methods such as Teflon-based electrocatalysts for electrode layers 1, 3, such as cathodes, have resulted in mass transport losses.
There remains a need for improved methods, which are less time-consuming and relatively inexpensive, to reduce surface roughness of electrode layers in electrochemical cells, achieve a better bond with thin membranes and prevent the onset of cracks.